1. Field of the Invention
The present invention relates to a semiconductor device using a crystalline semiconductor thin film formed on a substrate having an insulating surface.
Incidentally, in the present specification, any of a thin film transistor (hereinafter referred to as a TFT), a semiconductor circuit, an electrooptical device, and an electronic equipment are included in the category of the semiconductor device. That is, any device capable of functioning by using semiconductor characteristics will be referred to as the semiconductor device.
Thus, the semiconductor device recited in claims of the present application includes not only a single component, such as a thin film transistor, but also a semiconductor circuit or an electrooptical device formed by integrating such single components, and further, an electronic equipment having those as parts.
2. Description of the Related Art
In recent years, attention has been paid to a technique for constructing a thin film transistor (TFT) by using a semiconductor thin film (its thickness is about several tens to several hundreds nm) formed on a substrate having an insulating surface. With respect to the thin film transistor, the development thereof particularly as a switching element for an image display device (for example, a liquid crystal display device: LCD) has been hastened.
For example, in the liquid crystal display device, trials have been made to apply TFTs to any electric circuit, such as a pixel matrix circuit for individually controlling pixel regions arranged in matrix, a driving circuit for controlling the pixel matrix circuit, and a logic circuit (calculation circuit, memory circuit, clock generator, etc.) for processing data signals from the outside.
In the present circumstances, although a TFT using a noncrystalline silicon film (amorphous silicon film) as an active layer is put to practical use, a TFT using a crystalline silicon film (polysilicon film or the like) is necessary for an electric circuit required the performance of further high speed operation, such as a driving circuit or a logic circuit.
Conventionally, high temperature annealing has been required to form a polycrystalline silicon film having high crystallinity. Such a polycrystalline silicon film is generally referred to as high temperature polysilicon. For the purpose of forming the high temperature polysilicon film, it is necessary to prepare a substrate having high heat resistance so that the substrate can withstand a process temperature near 1000xc2x0 C. For that reason, in the present circumstances, a quartz substrate (according to circumstances, a silicon substrate) is used.
However, the quartz substrate has a high unit cost, so that the quartz substrate has problems of increasing the cost of manufacture, and further, increasing the cost of a product. Thus, in recent years, attention has been paid to a low temperature polysilicon film formed on an inexpensive glass substrate, and the research of the high temperature polysilicon film has been gradually declined.
The coefficient of thermal expansion of the quartz substrate is about 0.48xc3x9710xe2x88x926xc2x0 C.xe2x88x921, which is as small as about {fraction (1/10)} of the coefficient of thermal expansion of silicon (about 4.15xc3x9710xe2x88x926xc2x0 C.xe2x88x921). That is, stress is apt to occur between the quartz substrate and silicon, and peeling (film peeling) of silicon or the like is apt to occur at a heat treatment.
Moreover, since it is difficult to make the quartz substrate large, the use of a TFT using the high temperature polysilicon is limited to a liquid crystal display device with a size of about 1 to 2 inches in diagonal for a projection type projector or the like. That is, there is a problem that such a TFT can not be used for display devices of the several tens inch class, such as a display for a note-sized personal computer.
As a method of forming a crystalline silicon film on a glass substrate, there are known techniques disclosed in Japanese Patent Unexamined Publication No. Hei. 7-130652 and No. Hei. 8-78329 by the same assignee as the present application. The techniques disclosed in these publications use a catalytic element for facilitating crystallization of an amorphous silicon film, so that the formation of the crystalline silicon film having excellent crystallinity can be made by a heat treatment at about 500 to 600xc2x0 C. and for about 4 hours.
Particularly, the technique disclosed in Japanese Patent Unexamined Publication No. Hei. 8-78329 makes crystal growth almost parallel to the substrate surface by applying the above technique. The present inventors et al. refer to the formed crystallized region particularly as a horizontal growth region (or lateral growth region).
However, even if a driving circuit is constructed by using such TFTs, the circuit does not still reach the state in which the required performance is completely satisfied. Particularly, in the present circumstances, it is impossible to construct a high speed logic circuit requiring an extremely high speed operation ranging from megahertz to gigahertz by conventional TFTS.
The present inventors have repeated various processes of trial and error to improve crystallinity of a crystalline silicon film (called a polysilicon film) including crystal grain boundaries. A semiamorphous semiconductor (Japanese Patent Unexamined Publication No. Sho. 57-160121), a monodomain semiconductor (Japanese Patent Unexamined Publication No. Hei. 8-139019), and the like can be sited.
The common concept of semiconductor films disclosed in the above publications is to make the crystal grain boundaries substantially harmless. That is, the most important object is to substantially eliminate the crystal grain boundaries to cause the movement of carriers (electrons or holes) to smoothly move.
However, it can be said that even the semiconductor film disclosed in the above publications is insufficient to carry out the high speed operation required by a logic circuit. That is, in order to realize a system-on-panel having a built-in logic circuit, the development of a completely novel material is required.
The present invention has been made in order to satisfy the above requirements, and an object thereof is to realize a semiconductor device having extremely high performance, which can construct such a high speed logic circuit as can not be manufactured by conventional TFTs.
In order to achieve the above object, according to a first aspect of the present invention, a semiconductor device comprises a glass substrate having a distortion point of not lower than 750xc2x0 C., an insulating silicon film formed on at least a front surface and a back surface of the glass substrate, and a TFT including a channel formation region of a semiconductor thin film made of a collective of a plurality of rod-like or flattened rod-like crystals and formed on the insulating silicon film, and the semiconductor device is characterized in that the plane orientation of the channel formation region is roughly {110} orientation, and not less than 90% of crystal lattices have continuity at crystal grin boundaries.
According to another aspect of the present invention, a semiconductor device comprises a glass substrate having a distortion point of not lower than 750xc2x0 C., an insulating silicon film formed on at least a front surface and a back surface of the glass substrate, and a TFT including a channel formation region of a semiconductor thin film made of a collective of a plurality of rod-like or flattened rod-like crystals and formed on the insulating silicon film, and the semiconductor device is characterized in that the plane orientation of the channel formation region is roughly {110} orientation, and not less than 90% of lattice stripes observed to cross crystal grain boundaries are linearly continuous between different crystal grains forming the crystal grain boundaries.
According to still another aspect of the present invention, a semiconductor device comprises a glass substrate having a distortion point of not lower than 750xc2x0 C., an insulating silicon film formed on at least a front surface and a back surface of the glass substrate, and a TFT including a channel formation region of a semiconductor thin film made of a collective of a plurality of rod-like or flattened rod-like crystals and formed on the insulating silicon film, and the semiconductor device is characterized in that an electron beam diffraction pattern observed when the channel formation region is vertically irradiated with an electron beam has regularity peculiar to {110} orientation.
According to still another aspect of the present invention, a semiconductor device comprises a glass substrate having a distortion point of not lower than 750xc2x0 C., an insulating silicon film formed on at least a front surface and a back surface of the glass substrate, and a TFT including a channel formation region of a crystalline semiconductor thin film formed on the insulating silicon film.
According to still another aspect of the present invention, a semiconductor device comprises a glass substrate having a distortion point of not lower than 750xc2x0 C., an insulating silicon film formed on at least a front surface and a back surface of the glass substrate, and a TFT including a channel formation region of a high temperature polysilicon film formed on the insulating silicon film.
Still another aspect of the present invention is characterized by comprising a glass substrate having a distortion point of not lower than 750xc2x0 C., an insulating silicon film formed on at least a front surface and a back surface of the glass substrate, and a TFT including a channel formation region of a crystalline semiconductor thin film formed on the insulating silicon film. Yet another aspect of the present invention is characterized by comprising a glass substrate having a distortion point of not lower than 750xc2x0 C., an insulating silicon film formed on at least a front surface and a back surface of the glass substrate, and a TFT including a channel formation region of a high temperature polysilicon film formed on the insulating silicon film.
The following three points can be enumerated as the important structural conditions of the present invention.
(1) A glass substrate (glass substrate having a distortion point of not lower than 750xc2x0 C.) having such heat resistance that the substrate can withstand the temperature of not lower than 750xc2x0 C. is used as a substrate.
(2) The outer surface (at least the front surface and the back surface, preferably all surfaces) of the high heat-resistant glass substrate is protected with an insulating silicon film.
(3) A crystalline semiconductor thin film excellent in conformity of crystal grain boundary is provided on the high heat-resistant glass substrate covered with the insulating silicon film.
In order to form the crystalline semiconductor thin film excellent in the conformity of crystal grain boundary developed by the present inventors, a heat treatment at a temperature exceeding 700xc2x0 C. is required. The details of this forming method will be described in the section of xe2x80x9cDetailed Description of the Inventionxe2x80x9d.
For the above reason, it is necessary to use a substrate having a distortion point of not lower than 750xc2x0 C. As such a substrate, although a quartz substrate is common, since the quartz substrate is expensive, the total cost is increased. Moreover, the coefficient of thermal expansion is 0.48xc3x9710xe2x88x926xc2x0 C.xe2x88x921, and is as small as about {fraction (1/10)} of the coefficient of thermal expansion of silicon (about 4.15xc3x9710xe2x88x926xc2x0 C.xe2x88x921). That is, stress is apt to occur between the quartz substrate and silicon, and peeling (film peeling) of silicon or the like is apt to occur at heat treatment.
Then, in the present invention, a crystallized glass having a distortion point of not lower than 750xc2x0 C. (typically 950 to 1100xc2x0 C., preferably 1000 to 1050xc2x0 C.) and having high heat resistance is used as a substrate. Since the crystallized glass can be made thinner than quartz, the cost of manufacture of a liquid crystal module or the like can be kept inexpensive. Moreover, because of the glass substrate, it is possible to make the substrate large, and it is also possible to design a reduction in costs by formation of plural products from one substrate according to multiple-face taking.
Further, the coefficient of thermal expansion can be easily changed by adjusting the constituents of the crystallized glass. Thus, it is easy to select the coefficient of thermal expansion near the coefficient of thermal expansion of the crystalline semiconductor thin film.
The present inventors aim at obtaining a system-on-panel, and realizing an inexpensive and high performance electronic equipment. For the purpose of actively using the merits, it is greatly more effective to use the inexpensive crystallized glass than the expensive quartz substrate.
However, since the crystallized glass has various constituents, there is a fear of outflow of constituents in the manufacturing steps of a semiconductor device. Thus, it is important to protect the crystallized glass with an insulating film (an insulating silicon film is preferable in view of affinity to a crystalline silicon film). For that purpose, in the entire process, it is necessary to protect at least the front surface and the back surface of the crystallized glass with the insulating film.
Since the side surface of the crystallized glass has a very small area on the whole, even if the side surface is exposed, a serious problem does not occur. However, it is needless to say that it is most preferable to completely cover the front surface, the side surface, and the back surface with the insulating film to completely prevent the outflow of the constituents.
However, a portion on which the insulating film is not formed occurs at a substrate supporting portion (pusher pin and the like) used in the film formation of the insulating film. However, since this portion is very small as compared with the total area, a problem does not occur.
In view of the above, the present inventors have reached the structure of the present invention that a crystalline semiconductor thin film excellent in conformity at crystal grain boundaries is provided on a high heat-resistant glass substrate in which the outer circumferential surface (preferably all surfaces) of the substrate is protected with an insulating silicon film.